Win7 with latest Opera update as of 12-14-2020:
I've experienced the same frustrating problem over last few days. I drilled down to a glitch in the Sync feature (disappointing). Once I signed in, synced, closed browser and re-launched it went: crash-attempted auto recover-crash... (persistently).
Could not get rid of problem with complete re-install. Tried a number of deletions in profile folder with only temporary results. Finally deleted whole profile in \Opera Stable\ folder (see path below) and was able to start from scratch with fresh install. It was painful to add back all of my preferences, custom pinned tabs, and extensions again (and again). Then closed and opened browser several times without issue. Tried logging in and syncing then logging out of sync and closing browser... Bang! Same crash issue.
Solution:
After building back my custom experience, I followed this procedure using Windows Explorer:

1. Followed path "C:\Users[User Name]\AppData\Roaming\Opera Software\Opera Stable\Preferences"
2. Copied and pasted "Preferences" (pasted as "Preferences - copy")
3. Renamed to "Preferences.12-14-20"
4. Opened browser, logged into sync, let sync, and logged out.
5. Closed and reopened Opera... Crash!
6. Followed path to "Preferences" file and deleted.
7. Copied and pasted "Preferences.12-14-20" ("Preferences.12-14-20 - Copy")
8. Renamed to "Preferences"
9. Opened Opera and all of my custom pinned tabs, extensions, and preferences reappeared and functioned properly. Hot Dog!!

Suggestions:
a) Make a new (dated) copy of "Preferences" file after a fresh and properly operating install and after any subsequent customization.
b) If you want a copy of your bookmarks on the Opera server so that your android devices can sync with them you'll want to run steps 4 - 9, until of course the Opera folks fix the glitch.
Best of luck.

@lt-ii You might want to do a complete uninstall; reboot; then in windows explorer go back to the path in step #1:
"C:\Users[User Name]\AppData\Roaming\Opera Software\Opera Stable"
and completely delete the contents in the "Opera Stable" folder including all sub folders.
You may find that the installer will work again after that.
Good luck.

@lt-ii I posted a fix a couple of hours ago and joanneleon used it to fix a variation of the problem. Good Luck.

Win7 with latest Opera update as of 12-14-2020:
I've experienced the same frustrating problem over last few days. I drilled down to a glitch in the Sync feature (disappointing). Once I signed in, synced, closed browser and re-launched it went: crash-attempted auto recover-crash... (persistently).
Could not get rid of problem with complete re-install. Tried a number of deletions in profile folder with only temporary results. Finally deleted whole profile in \Opera Stable\ folder (see path below) and was able to start from scratch with fresh install. It was painful to add back all of my preferences, custom pinned tabs, and extensions again (and again). Then closed and opened browser several times without issue. Tried logging in and syncing then logging out of sync and closing browser... Bang! Same crash issue.
Solution:
After building back my custom experience, I followed this procedure using Windows Explorer:

1. Followed path "C:\Users[User Name]\AppData\Roaming\Opera Software\Opera Stable\Preferences"
2. Copied and pasted "Preferences" (pasted as "Preferences - copy")
3. Renamed to "Preferences.12-14-20"
4. Opened browser, logged into sync, let sync, and logged out.
5. Closed and reopened Opera... Crash!
6. Followed path to "Preferences" file and deleted.
7. Copied and pasted "Preferences.12-14-20" ("Preferences.12-14-20 - Copy")
8. Renamed to "Preferences"
9. Opened Opera and all of my custom pinned tabs, extensions, and preferences reappeared and functioned properly. Hot Dog!!

Suggestions:
a) Make a new (dated) copy of "Preferences" file after a fresh and properly operating install and after any subsequent customization.
b) If you want a copy of your bookmarks on the Opera server so that your android devices can sync with them you'll want to run steps 4 - 9, until of course the Opera folks fix the glitch.
Best of luck.